Recently, engineers are interested in optical recording media capable of high-density recording and of erasing and rewriting the recorded information. Such rewritable optical recording media include media of the phase change type wherein a laser beam is irradiated to a recording layer to change its crystallographic state to record a bit of information whereupon the information is read by detecting a change of reflectance of the recording layer due to the state change. The phase change optical recording media have the advantages that they can be overwritten by modulating the intensity of a single light beam and the drive requires a simpler optical system than that used for magneto-optical recording media.
In recording layers of the phase change type, chalcogenide materials such as Ge--Te and Ge--Sb--Te materials are often used because of a substantial difference of reflectance between crystalline and amorphous states and the relatively high stability of an amorphous state. Additionally, it was recently proposed to apply compounds known as chalcopyrite to the phase change recording layer. The chalcopyrite compounds have been widely studied as compound semiconductor material and applied to solar batteries and the like. The chalcopyrite compounds have a composition represented by Ib-IIIb-VIb2 or IIb-IVb-Vb2 according to the notation of the Periodic Table and are configured to have two stacked diamond structures. The structure of chalcopyrite compounds can be readily determined by x-ray structural analysis. Their fundamental characteristics are described in, for example, Monthly Physics, vol. 8, No. 8, 1987, p. 441 and Electrochemistry, vol. 56, No. 4, 1988, p. 228. Of these chalcopyrite compounds, it is known that AgInTe.sub.2 can be used, after dilution with Sb or Bi, as the recording layer material in optical recording media adapted for operation at a linear velocity of about 7 m/s. JP-A 3-240590, 3-99884, 3-82593, 3-73384 and 4-151286 disclose phase change optical recording media using such chalcopyrite compounds. Besides, JP-A 4-267192, 4-232779 and 6-166268 disclose phase change optical recording media wherein an AgSbTe.sub.2 phase is created as the recording layer crystallizes.
In general, information is recorded in phase change optical recording media by first conditioning the entire recording layer to be crystalline and irradiating a laser beam of a sufficient high power (recording power) to heat the recording layer at or above its melting point. At every spot where the recording power is applied, the recording layer is melted and then quenched, forming an amorphous recorded mark. The recorded mark can be erased by irradiating a laser beam of a relatively low power (erase power) so that the recording layer is heated to a temperature from its crystallization temperature to lower than its melting point. At the recorded mark where the erase power is applied, the material is heated at or above the crystallization temperature and then slowly cooled, resuming crystallinity. Consequently, the phase change optical recording medium can be overwritten by modulating the intensity of a single light beam.
As compared with magnetic recording media, phase change optical recording media and other optical recording media generally have a high recording density. The recent need to process a vast quantity of information as in images requires to further increase the recording density. The recording density per unit area can be increased by either narrowing the track pitch or reducing the space or blank between recorded marks to increase a linear recording density. However, if the track density or linear recording density is too high relative to the beam spot of reading light, the carrier-to-noise (C/N) ratio lowers, eventually to a level where signals are unreadable. The resolution upon signal readout is determined by the diameter of a beam spot. More illustratively, provided that the reading light has a wavelength .lambda. and the optical system of the reading equipment has a numerical aperture NA, the readout limit is generally given by a spatial frequency 2NA/.lambda.. Accordingly, reducing the wavelength of reading light and increasing the NA are effective means for improving the C/N and resolution upon readout. A number of technical studies that have been made thus far reveal that many technical problems must be solved before such effective means can be introduced.
Under the circumstances, JP-A 2-96926 discloses a recording carrier having a layer of non-linear optical material capable of ultrahigh resolution. The non-linear optical material is a material whose optical characteristics vary with incident radiation. Such changes include changes of transmittance, reflectance and refractive index as well as a change of the shape of the layer. By irradiating a reading light beam to the information-bearing surface through the non-linear optical material layer, smaller areas of the object can be read out.
The above patent discloses a breaching layer as the non-linear optical material layer. The breaching layer increases transmittance as the intensity of incident radiation increases. Gallium arsenide, indium arsenide and indium antimony are exemplified as the material used in the breaching layer. The layer of such non-linear optical material requires for reading light to have a high energy density since all absorption centers must be excited.
JP-A 5-89511, 5-109117, and 5-109119 disclose optical disks comprising a substrate having optically readable phase pits formed therein and a layer of a material that changes its reflectance with temperature. The reflectance changing material layer, through approximately the same action as the non-linear material layer in the above-referred JP-A 2-96926, is effective in achieving a high resolution beyond the readout limit governed by the wavelength .lambda. of reading light and the numerical aperture NA of an objective lens. The material layer requires for reading light to have a high power since a change from crystal to liquid or from amorphous to liquid is necessary upon readout.
JP-A 7-169094 discloses an optical recording medium comprising a phase change recording layer, a mask layer, and an intermediate dielectric layer disposed therebetween. The mask layer has substantially the same function as the non-linear material layer in JP-A 2-96926 and is formed of a phase-changeable material. In this optical recording medium, upon readout, the mask layer is melted to reduce the imaginary part of its complex index of refraction to the range of 0.25 to 1.0 so that microscopic recorded marks can be read out through the molten portions of the mask layer.
It was recently proposed to improve the readout limit by constructing a structure analogous to the ultrahigh resolution medium described in JP-A 2-96926 and placing the non-linear optical material layer and the recording layer more closely so that near field light may be utilized, as reported in Appl. Phys. Lett., Vol. 73, No. 15, pp. 2078-2080, 1998. In this report, the non-linear optical material layer is formed of Sb, the recording layer is formed of Ge.sub.2 Sb.sub.2 Te.sub.5, and an SiN layer of 20 nm thick is sandwiched between the non-linear optical material layer and the recording layer. The report states that recorded marks of less than 100 nm can be read out. It is noted that in Example of JP-A 7-169094, the distance between the non-linear optical material layer (specifically the mask layer) and the recording layer, which corresponds to the thickness of the intermediate dielectric layer, is set at 180 nm, which is significantly greater than the distance between the non-linear optical material layer and the recording layer in Appl. Phys. Lett., Vol. 73, No. 15, pp. 2078-2080, 1998.
In the report of Appl. Phys. Lett., Vol. 73, No. 15, pp. 2078-2080, 1998, a laser beam is irradiated to the recording layer through the non-linear optical material layer to form recorded marks. A laser beam defines a spot having an intensity distribution approximate to the Gaussian distribution wherein the intensity decreases from near the center toward the periphery. Then in conventional phase change optical recording media, by using a recording light beam of approximate power, only the area closely surrounding the center of the beam spot can be heated to a temperature necessary for recording. A recorded mark which is smaller than the beam spot diameter can be formed in this way. However, we found that when a recording light beam is irradiated to the recording layer through the non-linear optical material layer, the area of the recording layer to which the laser beam is irradiated has an energy distribution largely departing from the Gaussian distribution, which renders it difficult to form microscopic recorded marks by utilizing only the area closely surrounding the center of the beam spot.
It is also presumed that in Appl. Phys. Lett., Vol. 73, No. 15, pp. 2078-2080, 1998, crystalline recorded marks are formed in the amorphous recording layer. With this method of forming crystalline recorded marks, it is substantially impossible to perform overwriting by modulating the intensity of a laser beam between the write power level and the erase power level. To enable overwriting, the medium must be constructed such that the recording layer is previously initialized or crystallized, and amorphous recorded marks are formed therein.
However, the formation of amorphous recorded marks requires a greater power than the formation of crystalline recorded marks since amorphous recorded marks are formed by once melting the recording layer. When a recording light beam of such high power is irradiated through the non-linear optical material layer, the non-linear optical material layer can be degraded. Also, if a recording light beam of high power is irradiated from the non-linear optical material layer side, there arises the problem that recorded marks located proximate the beam spot (that is, recorded marks pre-formed on a track in a medium and recorded marks on adjacent tracks) can be erased.
For initialization, a laser beam must be irradiated over the entire surface of the recording layer by means of a bulk eraser. If the laser beam is irradiated from the non-linear optical material layer side, the non-linear optical material layer is undesirably heated and thus deteriorated. Another problem is that the power required for initialization becomes greater than in conventional phase change optical recording media.
We actually prepared a medium and tested it for write/read operation in accordance with Appl. Phys. Lett., Vol. 73, No. 15, pp. 2078-2080, 1998. It was confirmed that recorded marks could be read out. However, as signal readout was repeated, the C/N ratio drastically dropped, rendering the readout impossible within a short time. The medium was found to have poor durability against readout. What we learned as the reason is that the recorded marks substantially erased within a short time since a reading light beam with relatively high energy was applied in order to induce an optical change in the non-linear optical material layer.